Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio of charged particles. It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and other chemical compounds. The MS principle consists of ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.
Many mass spectrometers use applied radio frequency (RF) fields to separate the ions based on their mass-to-charge ratio, to trap ions for reaction or future analysis, or to direct the ions to other parts of the instrument. The quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods. The quadrupole ion guide consists of four parallel rods made of metal or metalized ceramic or glass. Each opposing rod pair is connected together electrically, and a radio frequency (RF) voltage is applied between one pair of rods and the other. A direct current voltage is then superimposed on the RF voltage. Ions travel down the quadrupole ion guide between the rods. Only ions of a certain mass-to-charge ratio m/z will reach the detector for a given ratio of voltages: other ions have unstable trajectories and will collide with the rods. This permits selection of an ion with a particular m/z or allows the operator to scan for a range of m/z-values by continuously varying the applied voltage.
The quadrupole ion trap mass analyzer is another type of mass analyzer used in mass spectrometry. Ions are stored in an RF field where the trajectories of the ions are bounded within the confines of the analyzer. As the RF field is varied, the trajectories of ions with particular m/z values become unstable and the ions are ejected from the trap. This permits the selection of an ion with a particular m/z or allows the operator to scan for a range of m/z-values by continuously varying the applied voltage.
RF only ion guides are commonly used in many mass spectrometers. These guides may be used to separate the ions from neutrals, to cool the ions for later injection into the mass analyzer, or to trap the ions for reaction before injection into the mass analyzer, or to focus the ion beam into the mass analyzer. The guides may be in a variety of configurations, but quadrupole rods, hexapole rods, and octapole rods are commonly used. In all cases, the RF field with no DC component allows ions of all m/z to pass through the device.
The applied RF fields are typically in the range between 500 kHz to 5 MHz. This frequency range requires electrical isolation of metal components as well as low losses of the applied RF in the insulators, particularly at elevated temperatures. These constraints usually restrict the materials to relatively expensive machined parts, e.g. alumina or Vespel. Additionally, Vespel and other organic polymers have softening points that preclude the use of them at temperatures above 250 C. Most organic polymers also will emit chemicals into the vacuum chamber when hot. These chemicals lead to background contamination of the mass analyzer. Inorganic materials, such as alumina and glass typically have softening points greater than 500 C. The moldable ceramic putties can be used, but these parts require a post-molding firing to set the material. This firing can produce oxide formation on the metal component as well as excessive variability in the ceramic shape and size Alternatively, dry moldable ceramic materials, e.g. glass bonded with alumina and a mineral flux such as cryolite, may be used. This is done by overmolding the metal component with molten ceramic. The moldable materials can be lossy in regards to RF but are less expensive to incorporate into a final product because the dry moldable material does not require post-molding operations such as firing or machining.
Formulations of ceramics containing lead oxide are more transparent to RF energy and have lower softening points. The lower the softening point of the ceramic, the easier it is to mold the part. The less RF energy that is absorbed by the ceramic, the less temperature increase is induced in the ceramic. This is helpful because at the temperature of the ceramic increases, the absorption of the RF energy also increases. If the RF absorption is too high, either the part will overheat or the RF generator will not be able to maintain the required field. For mass spectrometry applications that use RF, the lower absorbance of parts containing lead oxide increases the operating temperature range of the part where thermal runaway is prevented. However, materials containing lead are being phased out due to environmental concerns and regulations, e.g. Reduction of Hazardous Materials directive, (RoHS).
There are moldable glass and ceramic formulations available that are RoHS compliant. As shown in FIG. 1, they possess unacceptably high RF losses at elevated temperatures as measured by the quality factor (Q). FIG. 1 also shows the novel materials described in this teaching that are RoHS compliant, have acceptable RF characteristics, and are physically robust, e.g. are mechanically hard and do not dissolve significantly in water.